NEAR-THRESHOLD STUDY OF THE POLARIZATION OF HE RESONANCE RADIATION USING AN ENERGY-SELECTED ELECTRON-BEAM

The linear polarization of resonance vacuum-ultraviolet vuv radiation emitted from collisionally excited helium atoms has been measured in t he threshold region below 25 eV. The apparatus consisted of a hemisphe rically analyzed electron beam interacting with a gas jet; the normall y emitted radiation was analyzed using reflection optics. The threshol d polarization:was measured to be unity, in agreement with arguments b ased on the conservation of angular momentum. The measured vuv excitat ion functions and corresponding polarization curve were compared to 11 -state R-matrix calculations of the excitation of the 2 (1)p state and excellent agreement was observed up to the n = 3 threshold. Negative- ion resonances and cascade had pronounced effects on the observed subl evel excitation functions and on the polarization.